1. The effects of excitatory postsynaptic potentials (EPSPs) on interspike intervals (ISIs) of neocortical neurons can be mimicked by pulse potentials (PPs) produced by current injection. The present report documents the dependence of the ISI shortening on the amplitudes of PPs and EPSPs and on the firing rate of the affected neuron. 2. In rhythmically firing neocortical neurons, the ISI shortenings caused by PPs arriving at specific times in the ISI can be described by a shortening-delay (S-D) curve. The S-D curve yields three measures of the PPs' ability to shorten the ISI: 1) the mean ISI shortening, S; 2) the maximum shortening, S(max); and 3) the effective interval, defined as the portion of the ISI in which the PP consistently shortens the ISI. For PPs ranging between 80 μV and 3.6 mV (and cells firing at 25 imp/s), the mean shortening increased with amplitude h as S (ms) = 1.2*h (mV)1.24 (r = 0.94; P < 0.01). S(max) increased linearly with amplitude as 4.9 ms/mV (r = 0.86, P < 0.01). The effective interval (as a percentage of the ISI) increased slightly with PP amplitude and had a mean value of 65 ± 21% 21 (mean ± SD). 3. S-D curves obtained with stimulus- evoked EPSPs varied with EPSP amplitude in a manner similar to those of PPs. The relations obtained for stimulus-evoked EPSPs were not statistically different from those obtained for PPs in the same cells. 4. To determine the effect of firing rate, PPs were applied while neurons fired at frequencies ranging from 8 to 71 imp/s. Both S and S(max) were approximately inversely proportional to the baseline firing rate (f(o)) and could be described as: S or S(max) = kf(o)m. The mean value of the exponent m (±SD) was 0.96 ± 0.25 for S and 1.2 ± 0.4 for S(max). These values were not statistically different from a value of 1 (1 group, 2-tailed t test). The effective interval did not vary significantly with firing rate. 5. The dependence of S on PP amplitude and baseline firing rate was incorporated into an expression for the average change in firing rate (Δf) produced by PPs occurring at rate f(s): Δf = 0.03 h1.24 f(s). The Δf increased with PP amplitude but did not vary significantly with the baseline firing rate. The values of Δf calculated from the S-D curves matched the values that were computed directly from the spike trains. 6. Comparison of Δf produced by brief pulses of current with Δf produced by the same net increment in current applied steadily revealed that the pulses produced significantly larger increases in the firing rate. Thus synaptic inputs arriving synchronously as transient pulses have a greater effect on the firing rate of cortical neurons than the same inputs arriving asynchronously.
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